The use of windmill blades has been known for facilitating varied activities such as pumping water, processing grain and supplying electricity. As early as the 1930s, horizontal axis wind turbines became popular and cost effective for the supply of electricity where the turbine drive train apparatus was mounted horizontal to the ground. It is desirable for wind generator blades to have high lift, minimum drag, and a broad operating envelope.
For a typical wind power generator, the kinetic energy extracted from the wind by wind turbine blade lift is converted into torque. The torque is converted into electricity using a drive train connected from the blades to a generator and power converter, which converts the resulting direct current electrical energy into smooth electrical power that can be transmitted on conventional power distribution circuits. Commercial wind turbines generally are on very tall towers (e.g., 300 ft.) in the atmospheric boundary layer, with lower shear gradients, and larger eddy sizes. The towers are typically 1.5 to 2.5 the blade length.
Commercial wind turbines typically have long, thin blades (e.g., 150+ ft.) which lead to very high tip speeds, and thus have to address significant differential loading and performance concerns over the length of the blades. As a consequence of their length, blades often approach the limits of material strength, and are sensitive to the magnitude of rotational and aerodynamic loading.
Wind turbines placed on lower towers (e.g., 75 ft) are most likely immersed in the turbulent mixing layer of the atmosphere. Wind turbine energy capture in its specific atmospheric regime and site location have effects on all the downstream design parameters, including the nacelle, blades, hub, drive train, generators, power quality, and controls.
Minor boundary layer disruptions (e.g., turbulence) interacting with a wind turbine blade appear as variations in effective angle of attack, and can lead to lateral flow and flow separation from the low pressure side of the blade. Such separation (e.g., stall) negatively impacts performance, and potentially can lead to failure of the blades (which are typically large flexible structures). Controlling flow separation is generally seen as desirable, and turbines are typically configured to avoid stall.
The improvement of wind turbine performance and the regulation of power generation by the rotation of the blades about their lateral axes in real time (to optimize their angles of attack) are known. Typical blades are configured to stall at angles of attack in the range of 10° to 12°, and the blades are operated at angles of attack in the range of 4° to 6°, thereby keeping the blades relatively safe from stalling in low to moderate turbulence conditions. Sometimes more conservative, low stall airfoil designs are used, which might provide for stall at 14° to 15°, and which have larger cross-sections that add structural strength. Such airfoils are less aerodynamically efficient, having lift-to-drag ratios in the range of 40 to 60.
It is understood there exists a need for improved blades and wind power generation systems. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.